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MRI (MR signals (T2 & T2* Relaxation (protons fall out of phase and…
MRI
MR signals
precession
When put in external magnetic field, the overall effect on a group of protons
- the group of spins moves in a particular way (like a spinning top)
speed of precession: Larmor frequency w0
- precession frequency is proportional to the strength of the magnetic field
Hydrogen nuclei
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proton is constantly spinning --> protons have their own magnetic fields (behave like bar magnets)
- magnetic field for each proton called magnetic moment (usually randomly oriented)
when external magnetic field B0 is applied: magnetic moments align either with/parallel or against/antiparallel to the external field
- preferred state is the one requiring least energy: parallel to B0
- more protons align with B0
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T2 & T2* Relaxation
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What are the causes for this loss of phase coherence?
- T2 relaxation results from slowly fluctuating magnetic field variations (inhomogeneity) within the local tissue
--> the internal inhomogeneity of spins/protons influencing other neighboring spins: SPIN-SPIN RELAXATION= T2 Relaxation
- also inhomogeneity within B0
T2*: effects that result from the combination of T2 relaxation and the dephasing that results from inhomogeneity in B0
T2 is the time taken for transverse magnetization to decay to 37% of its initial value --> spin-spin interaction governs the speed of T2 relaxation
Free water: rapidly moving small molecules relatively far apart--> less spin-spin interaction --> longer T2 (compared to water-based tissues with large macromolecules eg. Grey matter
For human tissue, transverse relaxation is always faster than longitudinal (T2 values are always less or equal to T1)
when we have a sequence of RF 180 degree pulses, we get a chain of spin echoes. Each subsequent echo will be of lower integrity due to T2 effects
Free Induction Decay
When RF pulse is switched off, T1 & T2 occur simultaneously and independently
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the sum magnetization vector constantly changes direction and magnitude --> spiralling down with time
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RF coils
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Purpose:
- to transmit RF energy to the tissue of interest
- to receive the induced RF signal back from the tissue of interest
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When switched on: B1 field combines with B0 to generate MR signals
- these are spatially localized snd encoded by the gradient magnetic fields
output signal picked up by the receiver coil --> digitalized --> reconstruction computer processor --> image yielded
Shim coils
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localization of MR signal required good homogeneity --> the more uniform the magnetic field the better
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Passive: placing sheets or little coins of metal at the edges of the magnetic bore (close to RF & gradient coils)
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T2-weighted images
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Dephasing caused by T2* relaxation (inhomogeneity within local magnetic field) is potentially reversible
Time to Echo TE: can be chosen by the researcher
- time between the delivery of the RF pulse and the receipt of the echo signal
stronger signal is received when short TE
BUT at short TE, differences in T2 have little influence on tissue contrast
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Gradient Echo
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magnetic field gradients produce a change in field strength --> change in Larmor frequency along a particular direction
loss of phase coherence can be reversed by applying another magnetic field gradient in the opposite direction
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short acquisition time but also greater signal loss than spin echo, also influenced by T2* effects
Gradient coils
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gradient coils represent the 3 orthogonal directions (x,y,z)
aren't cooled too much, only room temperature
magnetic field generated by them is superimposed on top of B0 --> main magnetic field strength varies along the direction of the applied gradient field
T1-weighted images
differences in the signal intensity between the tissues is predominantly due to differences in tissue T1 relaxation time
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choosing a long TR eg. 1500msec
- allows tissues to recover their longitudinal magnetization fully --> no real difference in the value of longitudinal magnetization between the tissues
--> no discernible difference in MR signal between the tissues --> NO SIGNAL CONTRAST
choosing a short TR eg. 20msec
- at the time of applying a second RF pulse, tissues with a long TR1 (fluid) will show less recovery in longitudinal magnetization than tissues with short T1 (fat)
--> tissues will have different signal intensities --> greater contrast
Magnets
Principles of Maxwell equations: when an electric current flows through a wire, a magnetic field is induced around the wire
we want to reduce the resistance to the flow of the electric current --> we can do that if a special metal conductor is cooled
- lower resistance = high electric currents produce high strength magnetic fields
- we use super conducting magnets
- the main magnet coils made of superconducting metal are cooled using cryogenic liquid helium
main magnetic coils generate a strong constant magnetic field B0
- strength measured in units of Tesla
- 1T= 20.000 times the Earth's magnetic field
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Equipment
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MRI Machine:
- special copper-lined examination room (Faraday shield) --> to keep electromagnetic noise out
- main magnet coils + 3 gradient coils + shim coils + RF transmitter coil
Image Construction
MR signal is localized in 3 dimension using 3 separate field gradients
- slice-selection gradient
- phase-encoding gradient
- frequency-encoding gradient
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